White light organic light emitting diode based on a luminescent material

Author: Chunyu Zhang, Xingyuan Liu_, Li Qin, Wanbin Zhu, Lijun
Source: Journal of Luminescence 122?C123 (2007) 590?C592
Summary:
Microcavity organic light emitting diodes (MOLEDs) have been studied to emit white light. The preparation structure is a glass/dielectric mirror/ITO/NPB/Alq/MgAg microcavity organic light emitting diode. Although there is only one luminescent layer, the device emits white light due to the device's two microcavity modes. By adjusting the response wavelength (488 nm and 612 nm) and intensity of the microcavity mode, the two microcavity modes can finally obtain a bright white electroluminescence with a maximum luminance of 16435 cd/m2 and a maximum lumen efficiency of 11.1 cd/A. In the International Lighting Association chromaticity diagram, the position of the color coordinates is (0.32, 0.34), and the color is very stable under different applied voltages.
1 Introduction Recently, white light organic light-emitting diodes have attracted a lot of attention due to their potential applications in illumination sources, liquid crystal backlights, and full color displays [1-3]. A method in which white light is generally used is to employ a multi-light-emitting layer in which each of the light-emitting layers emits different light. By color mixing, finally white light is obtained. At present, there is still a need for improvement in electroluminescence properties such as color stability and efficiency obtained by such a method for fabricating devices.
An optical microcavity structure is a structure that is at least one dimension in the same dimension as the wavelength of light [4-7]. In a microcavity device, only one or a few optical modes can interact with the material in the microcavity. Therefore, the light undergoes a more significant change in the interaction of the material in the microcavity than in the free space.
In the past few years, organic materials have achieved significant effects through the correction of multi-luminous properties of microcavity effects, such as in enhancing brightness and modulating color [5-9]. In organic light-emitting diodes, both monochromatic and white light can be modulated by microcavities. In order to obtain white light in MOLED, it is very important to have multiple microcavity modes in the emission spectrum of the broadband luminescent material, which can be achieved by increasing the length or structure of the cavity [9]. However, various luminescent materials have been used in the white light MOLED reported in the literature [9], and the electroluminescence properties have not been sufficiently reported. In this paper, we successfully prepared a bright white MOLED using a luminescent material in a simple two-layer structure.
2 Design and experiment
The structure of the MOLED device is glass/DBR/ITO/NPB/Alq/MgAg. ITO acts as the anode. The MgAg mirror serves as the cathode. 4-40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB) was used as a hole transport layer (HTL). Tris (8-hydroxyquinoline) aluminum (Alq) acts as an emissive layer (EML) and an electron transport layer (ETL). Distributed Bragg Reflectors (DBR) and MgAg are used as the two mirrors of the microcavity structure.

Figure 1: Molecular structure of organic materials used in devices

Figure 1 shows the molecular structure of NPB and Alq. Alq is an organic luminescent material that is often used for yellow-green light. Alq was chosen as the EML because it has a broadband photoluminescence spectrum with a peak emission at 515 nm. In order to achieve a white light MOLED, at least two microcavity modes capable of exciting light in the red and blue spectral regions of Alq are required, while most of the green light in Alq should be suppressed. Therefore, the DBR is designed to have the largest reflection coefficient at the 530 nm Bragg wavelength.

The MOLED point-to-luminescence spectrum is determined by the following formula:


Here Is the effective distance between the exciton forming region and the metal mirror surface, with They are the reflection coefficients of the DBR and the metal mirror. Is the total optical thickness of the microcavity, Is at The density of free space at the place. Through simulation, we decided that the device structure of MOLED is as follows:
Glass/DBR/ITO (194 nm)/NPB (93 nm)/Alq (49 nm)/MgAg (150 nm)
DBR and ITO are deposited by electron beam evaporation. All organic layers and metal MgAg (rate ratio 10:1) layers were prepared by vacuum deposition. The PL spectrum was measured by a Hitachi F-4500 spectrophotometer. Brightness and EL spectra were measured on an Optikon CCD spectrometer. The emission spectrum was measured by a Shimadzu UV-3000 spectrophotometer.
3 results
The emission spectrum of DBR is shown in Figure 2. In Figure 2, there is also the PL spectrum of the Alq film and the simulated EL spectrum of the MOLED. The PL spectrum of Alq has a peak at 515 nm and a full width at half maximum of 90 nm. The DBR emission spectrum has the largest reflection coefficient at 530 nm, which is approximately 66%. The simulated EL spectrum shows two microcavity modes at 489 nm (full width at half maximum of 18 nm) and 613 nm (full width at half maximum of 42 nm). In the chromaticity diagram of the International Lighting Association, the coordinate position is (0.27, 0.4), which belongs to the white light range.

Figure 2. Simulated EL spectrum (line) of MOLED and PL spectrum (dot) of Alq film and emission spectrum of DBR

The electric field distribution in MOLED and the refractive index of the material are shown in Figure 3. The refractive indices of Ta 2 O 5 , SiO 2 , ITO , NPB , Alq and MgAg were 2.0 , 1.46 , 2.05 , 1.84 and 1.74 , respectively. In order to maximize device brightness, the antinodes of the microcavity region should be located at 489 nm and 613 nm from the NPB and Alq interfaces.

Figure 3. Electric field distribution in MOLED and refractive index of material
The turn-on voltage of the MOLED is 3V. When an applied voltage is applied, the device emits light from the side of the glass. When the applied voltage is at 12V, the EL spectrum of the device is shown in Figure 4. The two resonance peaks are located at 488 nm and 612 nm, which is in good agreement with the model. The full width at half maximum of the peaks is 20 nm and 32 nm, respectively.

Figure 4. EL spectrum of MOLED at 12V

Figure 5. Luminous efficiency (open squares) and brightness versus current density for MOLED devices. Figure 5 shows the luminous efficiency and brightness versus current density for MOLED devices. The maximum brightness of the device is 16435 cd/m 2 and the maximum efficiency is 11.1 cd/A. When the luminance of the MOLED device is 100 cd/m 2 , the luminous efficiency, voltage, and current density of the device are 9 cd/A, 6 V, and 1.2 mA/cm 2 , respectively. The color of the device's illumination is (0.32, 0.34) in the International Lighting Association's 1931 chromaticity diagram. The luminescent color is also very stable at different voltages, mainly because of the simple structure of the device and the relatively stable luminescent material Alq.
4 Summary We have developed a white light MOLED device that is simple in structure and uses broadband Alq as a luminescent material. White light is achieved by controlling the microcavity structure, the maximum brightness of the device is 16435 cd/m 2 and the maximum efficiency is 11.1 cd/A. The color of the device's illumination is (0.32, 0.34) in the International Lighting Association's 1931 chromaticity diagram, and the luminescent color is very stable at different applied voltages. The above results show that the microcavity structure is an effective method for preparing white OLEDs.
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Source: China Semiconductor Lighting Network Compilation




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